Isotopic Enrichment

Organic chemicals and biochemicals are most often composed of hydrogen, carbon, oxygen, and nitrogen. For those elements, the less abundant isotopes, i.e., carbon-13 (1.1%), deuterium (0.01%), nitrogen-15 (0.36%), and oxygen-18 (0.2%), exist throughout nature. 

Standard practice in the stable isotope field is to report the isotopic enrichment of the stable isotope in any given molecule. Examples include D2O (99.8%), methyl iodide (D3, 99.5%), and acetic acid (13C2, 99%). 

Isotopic enrichment is the mole fraction of the isotope expressed as a percentage. Isotopic enrichment refers to specific sites within the molecule and thus are site-specific; however, isotopic enrichment can be reported as an aggregate value for compounds with more than one labeled site. Isotopic enrichment is not to be confused with species abundance, which is the percentage of molecules with the same isotopic composition. For example, methyl iodide (D3, 99.5%) does not mean that 99.5% of the molecules are methyl-D3. In fact, a D3-labeled molecule at 99.5% enrichment means that 98.5% of the molecules will be the CD3 species, and 1.5% will be CD2H.

For deuterated compounds, the species abundance for each isotopologue can be calculated using a binomial expansion if the isotope enrichment is known and assumed to be equivalent for all labeled sites.* Please refer to Figure 1 (see brochure), which illustrates the use of Pascal’s triangle to calculate species abundances for molecules containing up to six deuterons (e.g., D). D is defined as the fractional isotopic enrichment for deuterium (e.g., the percentage isotopic enrichment divided by 100), and H is the fractional abundance for hydrogen (e.g., H=1-D). For example, to determine the species abundances for a D3 molecule at 99% enrichment, one would refer to the row identified on the left as “D3,” which shows the possible isotopologues of a D3molecule (e.g., D3, D2H, DH2 and H3), with the calculated species abundance listed directly below. For example, the fractional species abundance of the D2H isotopologue at a 99% isotopic enrichment would be 3(0.99)2(0.01)=0.0294 which converts to a percent species abundance of 2.94%.

To illustrate this relationship, the following tables (see brochure) show the actual species abundance for molecules containing one to nine deuterium atoms at various enrichments. These relationships are valid with other stable isotopes, such as 13C, 15N, etc. The species abundances in the tables were calculated from the isotopic enrichment by use of a polynomial expansion, assuming random distribution. 

*In rare cases, the species abundance values presented may differ from those present in authentic, multi-labeled compounds because the isotopic enrichment may not be equivalent for all labeled sites due to the manufacturing process. 

Isotopic Enrichment and Species Abundance